A broken line in FIG. 15 indicates a theoretical thermal efficiency in an Otto-cycle engine, and it is known that the thermal efficiency is a function of only a compression ratio and is increased with an increase in the compression ratio. However, if the compression ratio is increased, a knocking is liable to be generated. For this reason, the actual circumstances are that the engine is operated with a thermal efficiency lower than the theoretical thermal efficiency by retarding the ignition timing to lower the maximum pressure and temperature. The knocking is liable to be generated in a lower-speed rotational range and hence, the thermal efficiency is significantly reduced in the lower-speed rotational range as illustrated in FIG. 15. This is one of factors which impede the development of the engine in which a sufficient torque is produced at a lower speed.
The knocking in the lower-speed rotational range means a phenomenon in which an unburned air-fuel mixture is adiabatically compressed into a higher temperature and a higher pressure by the expansion of a combustion gas in an already burned area with progression of a surface of flame after ignition, and such unburned air-fuel mixture is self-fired before arrival of a normal surface of flame. The lower the number of rotations, the lower the burning speed, and the longer the time expired up to the arrival of the normal surface of flame and hence, the knocking is liable to occur. In this case, if the knocking is avoided by retarding the ignition timing to lower the maximum pressure and temperature, as described above, a reduction in thermal efficiency is brought about.
Therefore, there is a proposed technique for inhibiting the knocking without reducing the thermal efficiency, wherein the burning speed is increased so that a normal combustion is completed in a time as short as possible. In this case, for example, a squish area is provided to supply an intense flow to the air-fuel mixture and to supply a swirl or a tumble flow to intake air. There is also a proposed technique in which in order to reduce the burning distance to complete the normal burning in a short time, a spark plug is disposed in a central portion of a combustion chamber, and the combustion chamber is formed into a shape near a spherical shape.
However, if the speed of burning of the air-fuel mixture is increased in order to inhibit the knocking, the following problem is encountered: A combustion gas having a higher temperature is brought into contact with a wall surface of the combustion chamber and an upper surface of a piston at a higher speed and hence, the heat loss is increased due to an increase in heat transfer rate to reduce the thermal efficiency as a result. For this reason, an enhancement in thermal efficiency corresponding to an increment in compression ratio cannot be obtained.